Silicon nanowires (SiNW) are important for many application, for example photovoltaics, Li-ion batteries, chemical/bio sensors and transistors. The ability to modify the silicon band gap and thus its optical properties by controlling the nanowire diameter is important for many of these applications. Large band gap changes are observed in silicon nanowires that have a diameter under 10 nm. However, high density nanowire arrays of silicon with such a small diameter are challenging to synthesize.
There exist multiple synthesis techniques for silicon nanowire arrays. For example, the VLS method (vapor-liquid-solid) is a mechanism for the growth of one-dimensional structures, such as nanowires, from chemical vapor deposition. The growth of one dimensional nanoscale crystals through direct adsorption of a gas phase on to a solid surface is challenging and generally very slow. The VLS mechanism circumvents this by introducing a catalytic liquid alloy phase (usually using noble metals) which can rapidly adsorb a vapor to supersaturation levels, and from which crystal growth can subsequently occur from nucleated seeds at the liquid-solid interface.
Other SiNW growth methods have also been developed. SiNWs have been grown using various chemical vapor deposition (CVD) methods, laser ablation, thermal evaporation/decomposition, supercritical fluid-liquid-solid (SFLS) synthesis, and wet methods like metal assisted etching or electrochemical anodizing. However, to date, there has been no simple, cost-effective method of making high density and aligned silicon nanowire arrays with nanowire diameters below 10 nm that have not required templating, catalysts, or surface pre/post-treatment.
VLS requires noble metal alloy catalysts that are very expensive. Additionally, the diameter of SiNWs depends on the size of metal catalyst particle (the “template” for growth). Thus, to synthesize small diameter SiNWs, one needs to use additional methods to reduce the size of the metal catalyst nanoparticles and to disperse them on the substrate. These techniques add to complexity and cost of the method. In addition, high density nanowire arrays are difficult to produce with VLS, reducing their advantageous surface area. Other methods that do not rely on VLS typically involve reaction catalysts that need to be removed from the nanowires after growth, or involve other post-treatment of the nanowires or pre-treatment of the substrate in order to achieve high density silicon nanowire arrays with below 10 nm diameter nanowires.